Early evolution of small body size in Homo floresiensis

Recent discoveries of Homo floresiensis and H. luzonensis raise questions regarding how extreme body size reduction occurred in some extinct Homo species in insular environments. Previous investigations at Mata Menge, Flores Island, Indonesia, suggested that the early Middle Pleistocene ancestors of H. floresiensis had even smaller jaws and teeth. Here, we report additional hominin fossils from the same deposits at Mata Menge. An adult humerus is estimated to be 9 − 16% shorter and thinner than the type specimen of H. floresiensis dated to ~60,000 years ago, and is smaller than any other Plio-Pleistocene adult hominin humeri hitherto reported. The newly recovered teeth are both exceptionally small; one of them bears closer morphological similarities to early Javanese H. erectus. The H. floresiensis lineage most likely evolved from early Asian H. erectus and was a long-lasting lineage on Flores with markedly diminutive body size since at least ~700,000 years ago.

already hardened substrate of the reddish brown palaeosol of Layer III.The upper boundary of Layer II is plane to slightly undulous (Fig. 2).Subangular to subrounded volcanic pebbles with a maximum diameter of 90 mm (average of 10 largest clasts) are scattered at the base of Layer II where it is thickest, with fewer smaller isolated pebbles occurring in the lower half of the layer.Rounded rip clasts of fine-grained muddy sediment are concentrated in certain intervals, but otherwise Layer II appears massive except for locally preserved faint laminations and early diagenetic Liesegang bands.Layer II contains moulds of freshwater gastropods, grassy leaf fossils, and vertebrate remains.Scattered stone artefacts also occur dispersed in Layer II 1 .
3) A 6.5 m-thick series of massive, tuffaceous clay-rich mudflow layers of variable thickness (Layers Ia-f) covers Layers II and III.The lowest three of these mudflows (Layers Id-f) have a limited thickness of less than 10 cm and are only developed in the western excavation area where they directly overlie the irregular top surface of Layer III unconformably.Towards the southeast, the mudflow Layer I-c directly overlies Layer II (Fig. 2).The boundary between the top of sandstone Layer II and the base of mudflow Layer I-c can be blurry but is relatively sharp in other places.
Up to 20 cm of the basal part of the lowest mudflow that overlies sandstone Layer II contains rounded muddy rip-up clasts and sandy material derived from the underlying sandy layer, as well as small fossils and fossil fragments, but higher up these coarser components rapidly decrease in abundance (see Fig. 2e).Fragmented freshwater diatoms are common in the mudflows.
Vertebrate fossils are widely represented in Layer II, with over 15,500 fossil vertebrate specimens excavated between 2013-2019 2 .Fossils identifiable as Stegodon comprise 34% of the assemblage.In addition, unidentifiable bone fragments (43% of the total assemblage, probably mostly representing Stegodon bone fragments 3 ), indicate a high fragmentation of the skeletal material.Murine rodent fossils comprise 19% of the assemblage, while the remainder of taxa represented (crocodiles, varanids, birds, frogs) each occupy less than 2% of the total.The lowest proportion is represented by hominin fossils (0.06%).
There is evidence for fluvial transportation of many of the vertebrate fossils prior to burial, with most specimens exhibiting some degree of fracturing.Apart from some excavation damage, old dry bone fractures with ragged surfaces are prevalent.Many bone fragments are weathered to some degree, while other bone and molar fragments tend to be rounded by fluvial transport 3  These combined observations suggest that most of the vertebrate fossils were considerably fragmented and water-transported prior to burial.Some of the broken larger bones may have been fractured due to trampling by Stegodon, whereas water transport also may have contributed to the fragmentation, followed by rounding during the water transport 3 .Variable weathering stages (not yet studied systematically) indicate that at least some of the bones were exposed on the surface for a considerable period of time prior to transport and burial 3 .Only in one instance (the articulated vertebrae and rib concentration of a single Stegodon individual), burial must have taken place not too long following the death of that individual.
The fossil assemblage associated with Layer II thus seems to represent a mixture of bones that accumulated over a longer period of time in a small streambed, and more complete fossils that accumulated in the same stream bed closer to the moment that the exposed bedding of the stream was sealed off by the sequence of mudflows.However, the vast majority of the animal carcasses were exposed on the surface for long enough to allow loss of soft tissues and disarticulation.Crocodiles and Komodo dragons have also probably contributed to the disarticulation of the Stegodon carcasses.The fining upward sequence of Layer II suggests that the large disarticulated Stegodon bones lying on top of Layer II accumulated in the river bedding, after a waning stage of waterflow had deposited the very fine sandstone at the top of Layer II, otherwise the sandy sediment would have at least partly covered these bones.
The fine-grained mudflow sediment that covered the stream bedding is thought to have originated from the Welas Caldera lake some four km to the northwest of Mata Menge, presumably by overflow of the lake during the volcanic activity of one of the volcanic cones situated inside the caldera 2 .The first mudflows arriving at the Mata Menge site eroded and incorporated some sandy material and small bone fragments from the streambed, but appear to have rapidly lost erosive strength as the large Stegodon bones on top of Layer II do not seem to have moved far, as evidenced by the two pelvis fragments belonging to the same individual.
The ten isolated hominin fossils are all from a narrow east-west extending area that runs parallel to the paleochannel axis (Fig. 2g; see also Extended Data Fig. 2

in ref. 1).
These ten fossils, mostly consisting of dental elements, represent at least four individuals (see Discussion), suggesting that the Mata Menge hominin assemblage, like the other fauna, was highly scattered and dispersed.The concentration of these hominin fossils in the upper 10 cm of Layer II, and their overall good preservation, also suggests limited reworking and a narrower time frame for the accumulation compared to the more evenly distributed other taxa throughout Layer II.Incorporation of the hominin fossils into the upper 15 cm of Layer II must have occurred relatively close to the moment that Layer II was buried by the Layer I mudflows.The Stegodon bones lying on top of Layer II may have reached their final destination around the same time.
Regarding the cause of death of the hominin and Stegodon individuals of which fossils had accumulated in the upper part or on top of Layer II, it is unlikely that this was related to volcanic activity inside the caldera lake that generated the mudflows, because articulated skeletons or parts of skeletons would be prevalent on the surface of Layer II, which is not the case.
the ridge provides for a more reasonable set of shape comparisons with respect to the particular methodology of the present study.
. However, complete skeletal elements, molars and tusks of Stegodon are also common.Some of the heavier Stegodon elements, including large fragments of long bones or tusks, occur at the base of Layer II, and in some places are surrounded by pebble lag concentrations.Other fossil specimens are present in the middle of Layer II.Some large Stegodon bones were recovered lying on top of Layer II and surrounded by mudflow sediment (see Fig. 2c).Generally, the Stegodon bones are not articulated, except for one instance where a single concentration of four ribs and three thoracic vertebrae were found articulated in Layer II (in quadrant P22; refer Fig. 2g for the excavation plan grid).Matching isolated Stegodon molars from the left and right side of single individuals have been found from 2 to10 m apart, and in addition the left and right pelvis shown in Figure 2c, lying on top of Layer II, also belonged to one Stegodon individual.Skeletal elements and teeth of smaller taxa occur throughout Layer II, notably isolated crocodile and Komodo dragon teeth, and murine rodent postcranial elements, jaws, and isolated incisors and molars.Small fossils of vertebrates are also incorporated in the basal ~20 cm of the lowest mudflow layer.

3 .
Principal component analysis (PCA) of cross-sectional shape of the distal humeral diaphysis.a, The first two principal components of distal diaphyseal shape among 40 fossil hominin humeri.Convex hulls define the fossil groups (Groups 1-3 = australopith, Group 4 = H.habilis, Group 5 = H.erectus s.l., Group 6 = H.naledi; see Supplementary Data 3 for more details).The dotted line connects LB1 to the average shape of the three sampled sections of SOA-MM9.Diaphyseal outlines depict shape variation along each component.SOA-MM9 is extreme along PC1 and is most similar in overall shape (based on Procrustes distance) to LB1 and to specimens of H. naledi (Group 6).b, PC1 and PC3 for the same data.Source data are provided as a Source Data file.

Table 1 . Crown dimensions of the new Mata Menge teeth as compared to those of Liang Bua H. floresiensis.
Measurements are in millimeters.*Scoredfollowingref.50.†Measured following the method of ref.51.‡Corrected for wear.

Table 2 . Comparative dental sample.
Specimens used for principal component analyses.†Metric data obtained from ref. 52.‡Metric data obtained from ref. 53.The other specimens were measured by Y.K. based on the original specimens or high-quality casts.Raw data for the modern human M3 are available in ref. 54. *